It is often desirable or necessary to measure the displacement of the movable part of a compliance device relative to the fixed portion of the device. This is particularly so for remote center compliance (RCC) devices which are passively compliant mechanisms that perform assembly, mating and insertion tasks. Various types of RCC devices are disclosed in U.S. Patent Nos. 4,098,001, 4,155,169, 4,337,579, 4,414,750, all incorporated herein by reference.
RCC devices are passively adjustable so that they quickly and easily accommodate for relatively small misalignments. These devices have now been instrumented so that, in addition to being passively adjustable for assembling misaligned parts, they are now also able to indicate a measure of the misalignment.
Presently RCC devices are instrumented by one of a number of displacement sensing systems, including inductive proximity sensors, linear photodiode arrays, differential photodiode pairs and dual axis lateral effect photoposition sensors. Often, these sensing systems employ instrumenting electronics on the movable part of the compliance device. Such electronics require signal and power wires which tend to interfere with the movable part and may adversely affect deflection characteristics of the RCC device. Assembling and disassembling may also be difficult. And a number of the displacement sensing techniques are not able to provide measurements in all six degrees of freedom in which the RCC device moves.
Various position sensing instruments have employed a magnetic pole in conjunction with a Hall Effect device to measure displacement. However, these devices have not provided satisfactory results in multiple degrees of freedom. They tend to experience considerable cross axis coupling wherein the measurement of the displacement in a particular degree of freedom is affected by movement in one or more other degrees of freedom. And because the magnetic field varies exponentially with distance, the sensed output also varies exponentially. As a result, the sensed displacement along or about one axis of interest is dependent upon the degree of displacement along or about an entirely different axis.